1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an LCD device equipped with a resin black matrix, capable of preventing the light leakage phenomenon by improving an OD (optical density) value of the resin black matrix.
2. Description of the Prior Art
As generally known in the art, an LCD device can be fabricated in a compact size with a lightweight and has low power consumption characteristics. Due to the above advantages, LCD devices have been extensively used for terminals of various information appliance or video appliances in place of cathode ray tubes (CRTs) . In particular, a thin film transistor (TFT) LCD device, in which a TFT is installed for each pixel as a switching device, can provide a large-sized display screen while realizing superior image quality similar to those of the CRT, so the TFT LCD device has been spotlighted for TV markets as well as notebook PC and monitor markets.
A typical twisted nematic (TN) mode LCD mainly includes an array substrate formed with a TFT and a pixel electrode, a color filter substrate formed with a black matrix and a color filter, and a liquid crystal layer aligned between the array substrate and the color filter substrate.
Herein, the black matrix prevents light from leaking to other areas, but allows the light to be introduced into an aperture area, while preventing the color mixing between adjacent color filters having different colors.
According to the conventional method, a chrome layer is deposited on a glass substrate, and then the chrome layer (opaque metal layer) is patterned through the photo and etching processes, thereby obtaining the black matrix. Otherwise, black resin is coated on a glass substrate, and then the glass substrate coated with the black resin is subject to the exposure and development processes, thereby obtaining the black matrix.
In detail, FIGS. 1 and 2 are sectional views illustrating conventional color filter substrates, in which FIG. 1 is a sectional view illustrating a color filter substrate equipped with a chrome black matrix, and FIG. 2 is a sectional view illustrating a color filter substrate equipped with a resin black matrix.
Referring to FIGS. 1 and 2, a chrome black matrix 2a or a resin black matrix 2b is formed on a glass substrate 1 while defining a pixel area. The pixel area defined by the chrome black matrix 2a or a resin black matrix 2b is provided with a red color filter 3, a green color filter 4 and a blue color filter 5. In addition, an over-coating layer 6 is formed on the entire surface of the glass substrate 1 so as to planarize the surface of the glass substrate 1 formed with the black matrix 2a or 2b and the color filters 3, 4 and 5, thereby forming a color filter substrate 10.
Herein, in view of an OD (optical density) value, which represents the light shielding level, the chrome black matrix 2a has the OD value identical to or higher than 4, so the chrome black matrix 2a has a superior light-shielding characteristic. In addition, since the chrome black matrix 2a can be fabricated with a thin thickness of about 1500 Å, the surface of the chrome black matrix 2a can be planarized to a predetermined level, so that the process for forming the over-coating layer 6 on the glass substrate 1 can be advantageously omitted.
However, since the chrome black matrix 2a has high reflectivity, a screen of the LCD equipped with the chrome black matrix 2a may be glossy when external light radiates onto the screen. In particular, chrome is not an environment-friendly material, so the usage of chrome is restricted. Thus, as a matter of fact, it is difficult use the chrome black matrix 2a. 
In contrast, the resin black matrix 2b as shown in FIG. 2 is easy to manage. In addition, since the resin black matrix 2b is not an environmental pollution material, the resin black matrix 2b is increasingly used instead of the chrome black matrix.
However, since the OD value of the resin black matrix 2b is significantly lower than that of the chrome black matrix 2a, the resin black matrix 2b must have the thickness more than 1 μm, thereby causing the problem when forming the resin black matrix 2b. In addition, even if the resin black matrix 2b has the thickness more than 1 μm, the OD value of the resin black matrix 2b is in a range of 3.0 to 3.5, so the light leakage phenomenon may occur.
Meanwhile, the OD value of the resin black matrix 2b may be improved if the thickness of the resin black matrix 2b further increases.
However, if the thickness of the resin black matrix 2b is increased, although it is possible to improve the OD value of the resin black matrix 2b, not only is the height of the resin black matrix 2b increased, but also the surface step difference may increase as shown in FIG. 3. Such an increase of the surface step difference may cause the rubbing defect in predetermined areas of the resin black matrix 2b, which are shown in FIG. 4 as dotted lines. That is, the rubbing defect may occur in the direction opposite to the rubbing direction.
That is, as shown in FIG. 5, which is a sectional view of FIG. 4, as the thickness of the resin black matrix 2b becomes enlarged, the surface step difference of the resin black matrix 2b is also increased, so that the rubbing defect may occur in the area located just below the step difference part, that is, the rubbing defect may occur at one side of the pixel.
If the rubbing defect occurs, liquid crystal is misaligned, thereby causing the light leakage phenomenon and degrading the image quality of the LCD.